1. Field of the Invention
The present invention relates to a tensioning apparatus for imparting a constant tension to a wire such as a wire to be wound on a solenoid coil by a coil winding machine. More particularly, the present invention is concerned with a tensioning apparatus which can vary and set the level of tension freely by using an electric motor.
2. Description of the Related Art
Tensioning apparatus of the kind described are used in, for example, a coil winding machine for the purpose of imparting a constant level of tension to a wire which is being unwound from a supply bobbin and wound on a coil bobbin.
An example of such a known tensioning apparatus is shown in FIG. 8.
The tensioning apparatus for a coil winding machine generally has a main body 110 which includes a main tension pulley (brake pulley) 114 which produces a braking torque, a swingable tension bar 115 for absorbing any fluctuation of the tension on the wire, and a pulley 117 provided on the free end of the tension bar 115. In operation, a predetermined level of tension is applied to the wire by cooperation between the main tension pulley 114 and the pulley 117. The main tension pulley 114 has a braking device which exerts a braking torque on the main tension pulley 114. The braking device includes a disk rotatable as a unit with the main tension pulley 114 and a band brake which acts on the outer periphery of the disk. The braking torque is adjustable by a mechanical control of the contact pressure between the band brake and the disk.
It is to be understood, however, that the pressure exerted by the band brake varies during long use of the brake.
It is therefore a current measure to apply the braking torque to the main tension pulley in a non-contact manner by means of a magnet and a magnetic disk which oppose each other. With this arrangement, the level of the braking torque can be freely set by adjusting the distance between the magnet and the magnetic disk.
This type of tensioning apparatus is shown in, for example, Japanese Patent Publication No. 11270/1988, Japanese Utility Model Publication No. 48609/1988, Japanese Patent Publication No. 22194/1989, Japanese Utility Model Publication No. 11419/1990 and Japanese Utility Model Publication No. 11420/1990.
A wire 103 which has passed the main tension pulley 114 is taken up by a bobbin 102 which is rotated at a high speed by a winding machine 101, through an idle pulley 118, the pulley 117 provided on the free end of the tension bar 115, an idle pulley 104 and a nozzle 105.
During the winding, the nozzle 105 is moved to the left and right as viewed in the drawing.
The tension bar 115 is swingable about a fulcrum of an axle 111 of the tension bar 115 and is urged in the clockwise direction by the force of a back tension spring 112 so as to pivot in accordance with the level of the tension thereby absorbing variations in the back tension, thus maintaining a constant level of tension at the position where the wire is wound.
The back tension exerted by the tension bar 115 is changeable or adjustable by replacing the tension spring or by changing the position of the tension spring after removal of the housing cover or by means of an externally operable knob.
The above-described tensioning device has a drawback in that a manual adjustment is necessary each time the specifications of the winding is changed, requiring laborious work for adjusting the level of the braking torque applied to the main tension pulley and adjustment of the back tension applied to the tension bar.
The necessity for the manual adjusting operations tremendously impedes the production efficiency of the conventional automatic coil winding apparatus in which a plurality of coils are wound at a time.
Another drawback of the conventional apparatus is that it has no means for amending any variation in the tension, when necessary, during the winding operation.